1. Field of the Invention
The present invention relates to a water-proof photographic support.
2. Description of the Prior Art
Photographic printing paper is prepared by coating a photographic emulsion on the surface of a support such as a baryta paper, a polyolefin-coated paper or the like. When pictures printed on printing paper are observed, light is not only reflected at the surface of the photographic emulsion layer, but also the light which has passed through the photographic emulsion layer reaches the support and is reflected at the surface thereof and passes back through the photographic emulsion layer again to reach the observer's eye. Since light is partly or completely absorbed in the photographic emulsion layer in this case, light and shade and color can be seen, and thus images observed. For example, the portions that appear white on a printing paper are not due to the photographic emulsion having a white color, but merely due to the transparency of the photographic emulsion; the light reflected upon the surface of the support is seen and thus the white color of the surface of the support is seen. Conversely, the portions that appear black on a printing paper are a result of the absorption of all of the light in the photographic emulsion layer; therefore, no light reaches the observer's eye so that the appearance is black. The portions that appear red or blue are due to the fact that, while light passes through the photographic emulsion layer and is reflected at the surface of the support and again passes through the photographic emulsion layer, the red or blue light components remain but the other components are absorbed by the photographic emulsion layer so that red or blue only is seen. Accordingly, the purpose of a photographic printing paper support is not only for simply supporting the photographic emulsion, but also has an important role in reflecting light which has passed through the photographic emulsion layer.
Recently, polyolefin-coated papers have been often used as a support for a photographic printing paper. When a polyolefin-coated paper is used as a support for a photographic printing paper, the reflection characteristics are important for the reasons mentioned above. Upon preparation of a polyolefin coated paper, white inorganic pigments such as titanium dioxide, zinc oxide, zinc sulfide or the like are added to the polyolefin layer on the side onto which a photographic emulsion is to be coated in order to enhance the reflectance of the surface of the support. In general, the reflectance of the surface is improved by increasing the amount of white pigments added to the polyolefin layer. However, when the amount of the white pigment in the polyolefin layer is increased, preferred results are not necessarily obtained as a support for a photographic printing paper, although the reflectance of the support is enhanced. In many cases, the reflectance is improved as the amount of white pigments is increased. However, the color of the support becomes yellowish so that the visual impression of whiteness is conversely reduced. If such a polyolefin-coated paper is used as a support for a printing paper, the yellowish color is further emphasized after coating a photographic emulsion thereon. Thus, images, particularly the highlight portions, become yellowish and image sharpness is lost so that the value as a printing paper is decreased. In addition, if the amount of white pigments in the polyolefin is increased, extrusion coating of the polyolefin can be conducted only with difficulty and the production cost is also increased. Therefore, a method for improving whiteness appearance without increasing the amount of white pigments is required.
One method for improving the whiteness of the support is a method which comprises adding a fluorescent whitening agent to the polyolefin layer (e.g., as disclosed in Japanese Patent Application (OPI) No. 6531/76 (corresponding to U.S. patent application Ser. No. 592,699, filed July 2, 1975 and to German Patent Application (OLS) No. 2,529,989)). The method comprises improving the whiteness by converting ultraviolet light striking the support into visible light and reflecting the visible light. However, ultraviolet absorbing agents are often used in a color photographic emulsion for the purpose of protecting the images therein. Therefore, ultraviolet light does not reach the surface of the support so that a whitening effect cannot be obtained.
Investigations on the surface reflection characteristics of a polyolefin-coated paper used as a photographic support have now been conducted in great detail and it has been found that in order to improve the whiteness appearance as a printing paper, it is insufficient to merely enhance the reflectance but necessary to maintain the color hue of the surface of the support within a definite range.
A method for measuring and expressing the color tone of paper, polyolefin-coated paper, or the like is set out in JIS Z8722 and JIS Z8730. In accordance with this method, the color tone is expressed by the three numerical values comprising L, a and b. The symbol L represents lightness and the larger the numerical value of L, the higher the lightness. The symbol a represents a reddish color and the larger the numerical value, the stronger is the reddish color; if the numerical value is negative, it means that a reddish color is insufficient, in other words, greenish color is predominant. The symbol b is an indication of a yellowish color and the larger the numerical value, the stronger is the yellowish color; if the numerical value is negative, it means that a yellowish color is insufficient and the color becomes bluish. Where a and b are both zero, the support is colorless. In general, the color tone is expressed by describing the values of a and b with the positions thereof on a graph in which a and b are taken on the ordinate and the abscissa, respectively. The numerical values of L, a and b used in the present invention are those measured using an automatic colorimetric color difference meter, AU-CH-1 Model manufactured by Toyo Rika Kogyo Co., Ltd. In colorimetry a standard board having the numerical values of L=92.2, a=-1.2 and b=+0.5 is used to adjust the colorimeter.
In general, the color tone of the polyolefin-coated paper to which white pigments are added becomes a positive numerical value for a and a positive numerical value for b. For example, where 10% by weight of titanium dioxide is added to polyethylene and such is extrusion-coated on a good quality paper having the values of L=97.2, a=1.6 and b=0.5 in a thickness of 0.040 mm, the color tone becomes L=96.7, a=0.6 and b=2.7, which has a quite strong yellowish color. In order to mask the yellowish color, a method comprising adding a blue coloring agent (bluing) is conventionally used. For instance, if 0.08% by weight of ultramarine blue which is a blue inorganic colorant is added to the above-described polyethylene containing 10% by weight of titanium dioxide and the polyethylene is extrusion-coated in a thickness of 0.040 mm, the color tone becomes L=93.7, a=-0.3 and b=-1.0 and the yellowish color is considerably reduced. If the amount of ultramarine blue added is further increased to 0.15% by weight, the color tone becomes L=92.5, a=-0.6 and b=-4.0 and the support has a bluish color which can hardly be said to be white any longer. Variations in the amount and the kind of blue coloring agents to various extents have now been studied but sufficiently satisfactory results could not be obtained. In addition, it was found that even if a combination in which both a and b values are zero is obtained, the visually observed color is somewhat yellowish. In addition, it was further found that if a photographic emulsion is coated onto such a support and development processing is performed, the gelatin in the emulsion becomes slightly yellowish, because of contamination due to chemicals. Furthermore, it was found that this yellowish color cannot be eliminated by merely using a bluing and if the bluish color of the support is strong, the printing paper appears to be greenish in color.
As a result of these investigations on the color tone of a photographic support, a support which visually appears extremely white has been developed where the values of a and b are in a specific range, but not zero.